A high-refractivity low-dispersion glass is briskly in high demand as a material for optical elements such as various lenses. As a glass having such optical properties, there are known a heavy tantalum flint glass TaSF17, and the like, which are described in “Glass Composition Handbook” written by Hiroshi OGAWA and Shinei OGAWA and issued by Japan Glass Product Society, 1991, page 106.